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Introduction to Linux - A Hands on Guide

This guide was created as an overview of the Linux Operating System, geared toward new users as an exploration tour and getting started guide, with exercises at the end of each chapter.
For more advanced trainees it can be a desktop reference, and a collection of the base knowledge needed to proceed with system and network administration. This book contains many real life examples derived from the author's experience as a Linux system and network administrator, trainer and consultant. They hope these examples will help you to get a better understanding of the Linux system and that you feel encouraged to try out things on your own.

we have a two node redhat HA cluster have some 5-6 LUNS on the SAN shared between the two.Both nodes have RHEL 5.5
Last week there was a kernel error in one of the node and failover was also not successful.
On review we come to know that both(multipath software) DM-MPIO and EMC PowerPath is running.
Can some body tell me that how to know which one is RH HA-cluster is using and which one should i stop so that this error does not occurs again.
I would also like to know how it can be stopped?

Which multipathing you are using depends on how you are referring and using the luns. Typically devices for powerpath are emcpower<a-z>. What devices you are using in your configuration should tell you which one you will need to keep.

FYI - Just to let you know putting urgent in the post won't get you help any faster. This is a volunteer based forum. In the event that you need assistance that quickly I would suggest that you contact EMC, and or Red Hat. That way you can use your support contracts to get priority support.

What devices your PV, VG, and LV are using would be related to how LVM is configured. LVM by default scans all the available devices looking for LVM related items. The problem with that is typically it will find /dev/sd{a..z} first and not multipath devices.

Use pvscan -vvv to determine how lvm is viewing your volumes. You can also look in lvm.conf to help determine how LVM should be behaving.

On the "Urgent" matter. I was trying to point out that if a problem is that severe your best route would be to exercise your support contracts to get quicker support.